Recently, large-capacity and exchangeable information recording mediums and disc drives used for the same are in wide use.
As conventional large-capacity and exchangeable information recording mediums, optical discs including DVDs and Blu-ray discs (hereinafter, also referred to as “BDs”) are well known. An optical disc drive apparatus performs recording or reproduction by forming tiny pits (recording marks) on an optical disc using laser light, and is suitable for large-capacity and exchangeable information recording. DVDs are characterized by using red laser light, and BDs are characterized by using blue laser light having a wavelength shorter than that of red laser light. Owing to this, BDs have a higher recording density to realize a larger capacity than DVDs.
Moreover, in recent years, multi-layer optical discs, namely, optical discs including a plurality of recording layers have been actively developed for further increasing the capacity. As DVDs and BDs, two-layer discs including two recording layers are already on the market. In the future, discs including a larger number of layers, such as a six-layer or eight-layer discs, are expected to be available.
FIG. 1 is a conceptual view of a three-layer optical disc including three recording layers. An optical disc 1 includes a substrate 2, and recording layers 3, 5 and 7 stacked on the substrate 2. Between the recording layers, intermediate layers 4 and 6 having a role of protecting the recording layers are provided, and a surface of the disc is covered with a cover layer 8 formed of a polycarbonate resin or the like. Optical laser light is directed from the side of the cover layer 8, which is the disc surface. The recording layer formed in contact with the substrate 2, i.e., the recording layer farthest from the disc surface is used as the reference layer. The recording layers are numbered orderly from the reference layer; i.e., the recording layer 3 is called L0 layer, the recording layer 5 is called L1 layer, and the recording layer 7 is called L2 layer. Hereinafter, in this specification, this manner of labeling will be adopted. This manner of labeling is merely an example, and there are occasions where the recording layers are called L0 layer, L1 layer, etc. from the recording layer closest to the disc surface.
FIG. 2 shows an area arrangement of a recording layer of a general optical disc. On a recording layer of a discus-shaped optical disc 1, a great number of tracks 11 are formed spirally. In each track 2, a great number of tiny blocks 12 are formed.
The width of the track 11 (track pitch) is, for example, 0.32 μm in the case of a BD. The block 12 is an error correction unit, and is a minimum unit by which a recording or reproduction operation is performed. The block 12 has a size of, for example, 1ECC (size: 32 kbytes) in the case of a DVD and 1 cluster (size: 64 kbytes) in the case of a BD. In terms of “sector” (size: 2 kbytes), which is the minimum unit of data of an optical disc, ECC and cluster are represented as 1ECC=16 sectors and 1 cluster=32 sectors.
Each recording layer includes a lead-in zone 13, a data zone 14 and a lead-out zone 15.
The data zone 14 is a zone where the user can record any information, for example, real-time data of music or video, computer data such as sentences, data bases or the like.
The lead-in zone 13 is positioned inner to the data zone 14 along a radial direction of the optical disc 1. The lead-out zone 15 is positioned outer to the data zone 14 along the radial direction of the optical disc 1. These zones include an area usable for recording management information on the optical disc 1 (DMA area or temporary DMA area), an area usable for adjusting a recording power, etc. (OPC (Optimum Power Control) area) and the like. These zones also have a role of preventing overrun of an optical pickup (not shown).
On such an optical disc, it is important to record information with an optimal recording condition (for example, recording power, and for example, pulse generation timing and pulse length called “strategy”, and the like) from the viewpoint of the recording and reproduction quality. For realizing this, trial recording (hereinafter, referred to as “recording calibration”) is widely performed in a prescribed area of the optical disc to find the optimal power and strategy (for example, Patent Document No. 1).
Recording calibration is performed in a recording calibration area (hereinafter, referred to also as an “OPC area”) included in the lead-in zone 13, the lead-out zone 15 or the like.
FIG. 18 shows a flow of a general recording calibration procedure.
Step 1801: The recording power is adjusted (hereinafter, referred to “power calibration”). Specifically, recording is performed while changing the recording power step by step (step-by-step recording), the recording quality of the recorded area (for example, modulation degree or BER (Block Error Rate), etc.) is measured, and an optimal power at which the recording quality is optimal is found.
Step 1802: The recording strategy is adjusted while the recording power is fixed (hereinafter, referred to as “strategy calibration”). Specifically, recording is performed while changing the pulse width with the recording power being fixed at the optimal power found in step 1801, the recording quality of the recorded area is measured, and an optimal strategy at which the recording quality is optimal is found.
On an optical disc such as a BD, data is recorded by irradiating the recording layer with laser light to change the recording layer, for example, from an amorphous state to a crystalline state. Since the state of the recording layer is changed in this manner, the transmittance and reflectance of the light (i.e., optical characteristics) are changed. Namely, a recorded area and a non-recorded area have different optical characteristics.
Therefore, when an optimal recording power is found by power calibration for an optical disc including two or more recording layers, the power found for one recording layer varies depending on the recording state of the other recording layer (either already recorded or unrecorded). Specifically, the following may occur, for example: recording is performed with an excessively large power while adjusting the recording power, and as a result, the area used for the calibration is destroyed, which influences the recording characteristic of the other recording layer corresponding to the destroyed area. Even if an excessively large power sufficient to destroy an area is not used, the transmittance varies by the magnitude of the power used for the recording. Especially, an area in which recording has been performed with a power not suitable to the optical disc allows the transmittance to vary more than, and is more likely to be influenced by the transmittance balance than, an area in which recording has been performed with a suitable power.
In a multi-layer disc, the transmittance of the laser light is varied by the recording state of the recording layer passed by the laser light. Therefore, the recording characteristics of a second recording layer counted from the surface on which laser light is incident or a recording layer(s) deeper than the second recording layer (farther from the laser light incidence surface than the second layer) are varied by the recording state of the recording layer closer to the laser light incidence surface, even on the same recording layer. Especially, in the case of power calibration by which recording is performed while changing the power, recording may possibly be performed at a recording power exceeding the range suitable to the optical disc in order to find an optimal recording power. An area in which power calibration has been performed is one of areas which influence the transmittance most.
Therefore, when laser light for recording information on a recording layer passes an area of another, shallower layer which has been used for power calibration, the laser light is significantly influenced by the transmittance balance of the another layer. From the recording quality of an area in which recording has been performed with a transmittance in such a varied state, an optimal power cannot be correctly obtained. As a method for avoiding these problems, a method of restricting the locations of OPC areas is well known (for example, Patent Document No. 2 and Patent Document No. 3).
FIG. 19 shows locations of the OPC areas in an optical disc including two recording layers. A first recording calibration area 200 provided in the recording layer L0 and a second recording calibration area 201 provided in the recording layer L1 are located at different radial positions. In addition, an area of the other layer existing between the recording calibration area and the disc surface (the laser light incidence surface) is secured as a reserved area 210 (unused area). In the example of FIG. 19, an area of the recording layer L1 at the same radial position as that of the first recording calibration area 200 (an area of the recording layer L1 corresponding to the first recording calibration area 200) is secured as the reserve area 210 (unused area). In the case of a write once medium on which recording can be performed only once, an unused area, i.e., a reserved area is in an unrecorded state. Therefore, regardless of the recording layer in which the recording calibration area is to be used, the laser light does not pass any recorded area before reaching the recording calibration area. Thus, the recording calibration area is not influenced by the transmittance of the other recording layer, and recording calibration can be always performed under the same conditions.
In consideration of a recording medium including more than two recording layers, Patent Document No. 3, for example, provides a case where an OPC area in an odd-numbered recording layer and an OPC area in an even-numbered recording layer adjacent to the odd-numbered recording layer are located at different radial positions. Namely, the OPC areas in odd-numbered recording layers or the OPC areas in the even-numbered recording layers may be located at the same radial position. Alternatively, the OPC areas may be located at different radial positions in all the recording layers.